Conventionally, development of a small-sized and low power consumption modulated light source has been expected. In such a modulated light source, it has been studied to apply a minute ring modulator using a silicon sub-micron optical waveguide.
FIG. 11 is a schematic view illustrating a schematic configuration of a conventional modulated light source using a ring modulator.
This modulated light source is constituted by including a distributed feedback type (DFB) laser 101, a ring modulator 102, a light power monitor PD 103, a wavelength controller 104, and a heater 105.
The light power monitor PD 103 detects a light power passing through the ring modulator 102. The wavelength controller 104 outputs a control signal of a wavelength of laser light based on the light power detected by the light power monitor PD 103. The heater 105 heats the ring modulator 102 according to the control signal of the wavelength controller 104 to adjust the wavelength.
In the modulated light source, the DFB laser 101 continuously oscillates to output laser light. The output laser light passes through an optical waveguide to be guided to the ring modulator 102, and a transmittance is modulated at the ring modulator 102.
At the ring modulator 102, a transmission spectrum thereof is a Lorentzian spectrum whose transmittance becomes the minimum at a resonant wavelength. At the ring modulator 102, a modulation signal is changed between a voltage V0 and a voltage V1 to change the resonant wavelength. The transmittance is thereby modulated, and it is possible to obtain intensity-modulated output light.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2012-64862
[Patent Document 2] Japanese Laid-open Patent Publication No. 2009-59729
The resonant wavelength of the ring modulator 102 varies according to a change of a circumferential optical path length of the ring modulator 102 caused by a fabrication error or a temperature change, and deviation from a wavelength of the oscillated laser light occurs. As illustrated in FIG. 12, the ring modulator 102 is heated by the heater 105 to increase a ring temperature so as to compensate the deviation, and adjustment of the resonant wavelength is performed.
However, in this case, there is a problem in which it becomes difficult to enable both securing reliability of the modulated light source and improving power efficiency of a wavelength adjustment mechanism and a modulation (reduction in a heater power and a modulation power). Reasons thereof are as described below.
A case is considered when a radius of the ring modulator is small as illustrated in FIG. 13A. In this case, a volume of the ring modulator becomes small, and the power consumption at the heater which is required to compensate the wavelength deviation at the temperature variation time decreases. Besides, an electrostatic capacitance to be a load of a drive circuit of the ring modulator becomes small, and therefore the modulation power also decreases. On the other hand, when compensation of an initial wavelength deviation is performed, it is necessary to compensate for a degree of a maximum spacing (FSR: free spectral range) of a ring resonant wavelength, and the FSR becomes large. Resulting from the increase in the FSR, the compensation amount of the wavelength increases, and as a result, an increase amount of the temperature of the ring modulator 102 increases to cause lowering of the reliability.
A case is considered when the radius of the ring modulator is large as illustrated in FIG. 13B. In this case, the FSR becomes small, the compensation amount of the wavelength decreases, the increase amount of the temperature of the ring modulator is reduced, and the reliability is secured. On the other hand, the volume of the ring modulator becomes large, and the power consumption at the heater which is required to compensate the wavelength deviation at the temperature variation time increases, further the modulation power increases.
Further, a problem caused by using the DFB laser 101 is not negligible. Namely, if a phase shift of a diffraction grating is eliminated to improve power efficiency of the DFB laser 101, yield is lowered. Conversely, when the phase shift is inserted to improve the yield, the power efficiency drops.